b162bb4270
Add powerpc64-ibm-aix as Tier-3 target
This is part of the effort mentioned in https://github.com/rust-lang/compiler-team/issues/553.
A reference to these options are definitions from [clang](ad6fe32032/clang/lib/Basic/Targets/PPC.h (L414-L448)) and [llvm](https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/PowerPC/PPCTargetMachine.cpp).
AIX has a system `ld` but [its options and behaviors](https://www.ibm.com/docs/en/aix/7.3?topic=l-ld-command) are different from GNU ld. Thanks to ``@bzEq`` for contributing the linking args.
rustc_target contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc dev guide.